1. Field of the Invention
The present invention is directed to a method and arrangement for the control of a dynamic scale system, referred to below as a dynamic scale, for weighing items during the transport thereof to a processing station, such as in a mail processing system.
2. Description of the Prior Art
xe2x80x9cMixed mailxe2x80x9d means an unsorted stack of postal items that, with respect to a side to be printed. The item letters is used below as representative of all other possible pieces of mail that differ in format, thickness and weight. Methods and arrangements are suited for users of mail processing systems both with a dynamic scale and postage-calculating postage meter machine as well as with dynamic postage-calculating scales and franking devices.
In a known, automatic mail processor Francopost 12000 marketed by Francotyp-Postalia AG and Co., a dynamic weighing station is arranged following a separating station. The letters applied as a stack are first automatically separated and are then accepted by the dynamic weighing station, and their weight is determined. The weight determination ensues with a precision that enables the allocation of the letters to the correct postage class. The postage is automatically transmitted to the connected franking unit, and is debited and printed onto the letter. When the letter arrives on the weighing pan and during the transport thereof, mechanical oscillations, having an amplitude dependent on the letter weight, the transport direction and speed, the distribution of the letter mass as well as the letter stiffness, are transmitted onto the weight sensor. The unwanted oscillations limit the precision of the measured result. Generally, a measuring precision of xc2x11 g is indicated in dynamic letter scales up to a letter weight of 1000 g. Given transport of heavy pieces of mail having a weight above 1000 g, the transience of the dynamic scale requires more waiting time as a consequence of the vibratory influences on the weighing system. All letters that are even heavier are usually separately weighed with a suitable static scale. The corresponding weight or postage then can be manually entered into the franking unit. The automatic execution is thus interrupted. Especially disadvantageous is the fact that the mail handler cannot predict whether a specific letter that is introduced in the a stack in common with the others will exceed this weight limit.
U.S. Pat. No. 4,778,018 discloses a dynamic letter scale wherein the weighing cell is resiliently suspended with respect to the transport mechanism. The transmission of oscillations from the transport mechanism onto the weighing cell thus is damped. For high weights, however, this damping is not sufficient in order to assure an adequate measuring precision. Moreover, such a resilient suspension produces a transient behavior of the weighing pan that lengthens the measuring time and thus reduces the letter throughput.
U.S. Pat. No. 5,014,797 discloses a non-modular device for automatic mail processing in which the dynamic weighing function is integrated. Shortening of the transport path and a letter throughput per time unit that is higher overall are intended to be achieved. In this solution, as well, however, the maximum weight is limited, given a predetermined measuring precision and processing speed. A product constructed according to this principle achieves a throughput of approximately 5000 an hour given a maximum weight of 500 g. For letters having a higher weight, the manufacturer offers an additional, static scale as an attachment. Thus, automatic processing of heavier letters is not possible.
U.S. Pat. No. 4,956,782 and British Specification 22 35 656 disclose a semi-dynamic weighing. The user station arranged upstream can be a scale and the user station arranged downstream is a postage meter machine. A continuously moving letter must be completely accepted by the scale for the measuring time required for the weight determination. The probability that the scale will determine an incorrect measured result is higher given mixed mail. The measuring time is increased dependent on the dimensions of the letter. A complicated control, which interprets the dimensions of the letter in advance, and control motors are required for this purpose. Alternatively, the weighing path could in fact be lengthened and a reject compartment could be provided. This, however, would increase the length of the overall mail processing system; which is not possible without extensive retrofitting.
According to European Application 514 107, a control means interrupts the transport given large letters and letters that are difficult to weigh until the measurement is stable. The measurement ensues in the static condition of the dynamic scale. A detector is arranged in the scale in the proximity of the downstream conveyor belt end, and only letters whose weight has been identified prior to this time pass through this detector. Measurement errors occur given a short weighing path and unevenly distributed mass in the letter or a high transport speed of high-mass letters. In the event of a sudden stoppage, moreover, the letter can slide from the weighing pan due to its mass inertia. The dimensions of the weighing pan are therefore designed somewhat larger or the transport speed is fixed lower. The throughput given mixed mail is correspondingly reduced.
German Published Patent 37 31 494 corresponding to U.S. Pat. No. 4,753,432 discloses weighing during a quiescent time, wherein the operation of the franking system and of the transport system are interrupted because the latter would otherwise produce an excessive vibration. The transport time from the weighing module to the postage meter machine is designed to be short. The speed cannot be arbitrarily increased, however, without increasing the risk of jams. The throughput performance that can be achieved is limited by pauses introduced into the executive sequence. The speed of the weighing is limited by the speed of the weight determination. This is particularly slowed for heavy letters, for which the measurement must be exact.
In contrast to semi-dynamic operation, dynamic operation is problematical for heavy letters. German OS 198 33 767 discloses a dynamic scale wherein the mass of the weighing pan is significantly decreased and its rigidity significantly enhanced. Short transient times for the overall measuring system result therefrom, as, consequently, do correspondingly higher letter transport speeds given dynamic operation.
With the goal of assuring an efficient mixed mail processing with a dynamic scale in conjunction with a newly developed postage meter machine, whereby the former implements a weight determination and the latter implements the calculation of the postage value, this known dynamic scale has a weighing pan for the letters that is implemented in lightweight structure but which is flexurally and torsionally rigid. This weighing pan is coupled to a weighing cell at the approximate location of the center of gravity of the weighing pan and of a letter with the highest allowed weight that is arranged centrally on the weighing pan. Negative effects of the load arm are minimized by the aforementioned coupling of the weighing pan to the weighing cell. An increase in the range of utilization is established by the improvement of the function properties of the dynamic scale.
A method for the control of a dynamic scale that can process mixed postal matter having different size and thickness and that can be operated in at least two operating modes is disclosed in German Patent Application DE 19860296.0. In the dynamic operating mode, a piece of mail is supplied, before the beginning of a first measuring time span, with a transport speed that is set to a predetermined value and independently of the letter format when the letter is located at the entry to the dynamic scale. Speed regulation is deactivated by the controller of the dynamic scale during the dynamic weighing in the measuring time span, so that a weight measurement ensues given a deactivated speed regulation. An activation of the speed regulation for the motor ensues again when the letter is located in the discharge of the dynamic scale. The letter subsequently passes through the postage meter machine or further processing stations of the mail processing system.
The scale can be switched from a dynamic to a semi-dynamic operating mode. If the scale identifies an invalid measured result, a reverse motion of the incorrectly measured letter is triggered within the scale. The scale stops the conveyor belt during the following semi-dynamic weighing until the scale has determined a correct measured result. However, the clock performance (throughout of letters per hour) is reduced because the scale only works in the static or, respectively, semi-dynamic operating mode for letters that were initially incorrectly weighed or for other pieces of mail.
An object of the present invention is to achieve an automatic processing of mixed mail with mailing having higher weight as well in the dynamic operating mode of a scale in order to increase the throughput of pieces of mail per hour. An optimally high proportion of the mail of a mail shipper should be weighed in the dynamic mode, with manual interventions being as few as possible during the mail processing.
The above object is achieved in accordance with the invention in a dynamic scale, and a method for operating a dynamic scale, wherein items to be weighed are transported on a transport device to a weighing cell, at which a measured weight value is obtained, and wherein the scale is controlled to operate in one of a number of operating modes of the transport device dependent on the measured weight value, which may include, if necessary, re-weighing the item before the item exits the dynamic scale.
It has been found that a dynamic weighing in two directions, downstream as well as upstream, is fundamentally possible with the inventive measuring arrangement. The measuring arrangement of the dynamic scale is coupled to a conveyor for the continuous transport of letters. The conveyor is called a transport device below. The transport direction is reversible, and thus a letter transport can be implemented upstream in the direction of the system entry location as well as downstream to the further processing station.
The piece of mailxe2x80x94referred to below as letterxe2x80x94is transported downstream to the further processing station with a first speed, whereby a weight measurement ensues which is interpreted for plausibility of the measured result. Given non-plausibility, i.e. the measured result of the letter weight is declared invalid, and a dynamic re-measurement of the weight is implemented. For this purpose, the letterxe2x80x94by reversing the transport direction of the transport devicexe2x80x94is transported with a second speed upstream into a predetermined position back to the letter entry.
During the return transport of the letter, the weight thereof is determined in a dynamic scale without an intermediate stop.
Given successful re-measurement for which the weighed result is plausible, the letter is carried downstream to the further processing station without further weighing. The discharge can ensue with a maximum speed prescribable for the dynamic scale, that is higher than the first and second speeds.
A further re-measurement can ensue when the measuring time is inadequate or when the re-measurement is not plausible. When reaching a predetermined position in the letter admission, the letter is transported downstream across the weighing system a second time with a third speed by again reversing the transport device, whereby the dynamic scale determines the weight of the letter with a further re-measurement during the transport. When the result of the re-measurement is plausible (valid), the letter is carried to the next processing station with the speed that is set for the system throughput without disturbing influences.
Another possible manner of operation of the invention is that, given a weighed value that is not plausible, the weighing system is left in the dynamic condition but no weight determination is implemented given return transport of the letter by reversing the transport direction of the transport devices in order to further damp the natural oscillations of the weighing system. A repetition of the weight determination of the letter during transport is implemented only when the letter has reached the predetermined position in the admission region and the transport of the letter downstream by the transport device has been resumed.
During the passage of the letter, sensors identify the letter in its respective positions. The implementation of the plausibility check is disclosed in German OS 198 60 296. The invention assures that a high proportion of the mail volume of the mail shipper ensues dynamically. Manual interventions are required only when other errors have occurred that have negatively influenced the oscillatory behavior of the weighing system such as, for example, impacts or improper handling of the system components.
It is of no significance for the realization of the invention whether the letters are transported horizontally or vertically. The invention is not limited to currently known weighing methods known. As stated above, the further processing station coupled to the scale receives weighed letters from the transport device of the dynamic scale. A control unit is connected to the weighing cell, the sensors and the transport device. The latter is fashioned for a bi-directional transport. The sensors identify the letter position in the transport path. The transport devices in the dynamic scale has a controllable drive, and the control unit includes a microprocessor that is connected to a program memory and to a non-volatile memory. The microprocessor is programmed such that the letter is transported upstream in the direction of the first processing station with a second speed or downstream in the direction of the further processing station with a second speed. Dependent on the result of the weight determination, the microprocessor switches from the first mode into a second operating mode. When the measured result is valid, the letter is discharged to a further processing device in the first operating mode. When the measurement is invalid, a switch is automatically made to a second operating mode. In the second operating mode, the letter is transported upstream in the direction of the first processing station with a second speed and is re-weighed, or given very poor vibratory behavior of the weighing system, is transported directly back to the predetermined position in the admission region without being weighed. The letter is dynamically re-weighed only after resumption of the transport direction of the letter downstream with a third speed. The microprocessor determines the respective magnitudes of the second and third speeds dependent on the extent to which the measuring arrangement with the weighing cell has already zeroed-in on the weight in the first operating mode.
It is provided that the second and third speeds are different in magnitude from, or the same as, the first speed but lower than a maximum speed with which a letter is carried downstream in the direction of the further processing station without weight determination.